Mobile (also referred to as “cellular”) telephones are becoming increasingly widely used globally. It is often important that they be made from materials that are able to withstand rigors of frequent use while still not interfering with the operation of the telephone and its ability to send and receive electromagnetic signals. Mobile telephone housings are a particularly demanding materials application. Mobile telephone housings comprise one or more components that can include the back and front covers, the backbone, and the antenna housing, depending on the design of the telephone.
In many cases the backbone is a frame onto which many of the components of the telephone, such as the screen, keypad, battery socket, microprocessors, other electronic components, antennas, etc., are mounted. In addition to providing structural support for the telephone and many of its components, the backbone may provide the primary protection of many of these components against impact. Covers may provide additional protection from impact and protect the backbone and internal components from contamination. Covers may also provide substantial or primary structural support for and protection against impact of certain components, such as screens and/or antennas. It is thus often important that the materials used for mobile telephone housings have high modulus and impact resistances.
Additionally, for optimal functioning of a mobile telephone it is often necessary or desirable that the housing material in the vicinity of the antenna be as transparent as possible to electromagnetic radiation having frequencies in the range of about to 40 MHz to 6 GHz and that the material's response to such electromagnetic radiation not vary significantly as a result of environment conditions such as temperature and relative humidity. Otherwise, the amplitude of the radio signals sent and received by the telephone can be affected, resulting in a lost or weakened connection or requiring increased power consumption to maintain a signal at a desirable level.
Thermoplastic polyamide compositions are desirable for use in making mobile telephone housings because of their good physical properties and that they may be conveniently and flexibly molded into a variety of articles of varying degrees of complexity and intricacy. However, many polyamides absorb significant amounts of moisture when exposed to atmospheric conditions, which may decrease the transparency of the compositions to 40 MHz to 6 GHz electromagnetic radiation. Typically, the greater the level of moisture absorbed by the composition, the less transparent the composition is to 40 MHz to 6 GHz electromagnetic radiation. Polyamide 12, for example, has good resistance to moisture absorption, but is insufficiently stiff for many mobile telephone housing applications.
Thus it would be desirable to obtain to make mobile telephone housings from a polyamide composition having a combination of good mechanical properties and low moisture absorption.